1. Technical Field
The present application relates to connection systems for spinal fixation devices, including cross connecting devices for spinal fixation bone anchors such as bone screws and hooks.
2. Related Art
The bones and connective tissue of an adult human spinal column includes more than twenty vertebrae coupled sequentially to one another by a tri joint complex. The complex includes an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The vertebrae are each anatomically categorized into one of four classifications: cervical, thoracic, lumbar, and sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve vertebrae are thoracic vertebrae, and connect to the lower spine comprising five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx).
The spinal column is highly complex in that it includes over twenty vertebrae coupled to one another for housing and protecting critical elements of the nervous system. These elements of the nervous system have seemingly innumerable peripheral nerves and circulatory bodies in close proximity to each other. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twisting in many different directions.
However, genetic or developmental irregularities, trauma, chronic stress, tumors and disease can result in spinal pathologies that either limit this range of motion, or threaten the critical elements of the nervous system protected by the spinal column. A variety of systems have been disclosed in the art which provide some degree of immobilization of the spine by implanting artificial assemblies in or onto the spinal column. These assemblies include anterior, posterior, and lateral assemblies. Lateral and anterior assemblies can be coupled to the anterior portion of the spine, typically between vertebral bodies. Posterior spinal fixation systems generally include a pair of rods, which can be aligned along an axis to which the bones are to be disposed, and which are then attached to the spinal column by spinal fixation bone anchors, such as pedicle hooks and/or pedicle screws. Hooks can be coupled to the lamina or attached to transverse processes, while screws can be inserted through pedicles. In order to provide enhanced torsional rigidity, these structures can include cross-connecting devices for coupling the rods together in a direction that is generally transverse with respect to the axis of the rods. These cross-connecting devices can be coupled directly to the rods themselves, or can be attached to the bone anchors.
A number of improvements to prior cross-connecting devices are desirable. For example, it is desirable to provide cross-connecting devices that are highly adjustable, for example both length-wise and angularly in several degrees of freedom.